- vkCreateSemaphore() create binary semaphore, unless timeline explicitly
requested. Previously, timeline semaphore was always created when
exportObjectType == VK_EXPORT_METAL_OBJECT_TYPE_METAL_SHARED_EVENT_BIT_EXT.
- vkCreateSemaphore() record error when MTLSharedEvent imported or marked for
export, and MoltenVK configured for VkSemaphore using MTLFence or CPU emulation.
- MVKTimelineSemaphore elide unused constructor arguments.